Perl Interview Questions

• Perl is a stable, cross platform programming language.

• Though Perl is not officially an acronym but few people used it as Practical Extraction and Report Language.

• It is used for mission critical projects in the public and private sectors.

• Perl is an Open Source software, licensed under its Artistic License, or the GNU General Public License (GPL).

• Perl was created by Larry Wall.

• Perl 1.0 was released to usenet's alt.comp.sources in 1987

• At the time of writing this tutorial, the latest version of perl is 5.16.2

• Perl is listed in the Oxford English Dictionary.

• Perl takes the best features from other languages, such as C, awk, sed, sh, and BASIC, among others.

• Perls database integration interface DBI supports third-party databases including Oracle, Sybase, Postgres, MySQL and others.

• Perl works with HTML, XML, and other mark-up languages.

• Perl supports Unicode.

• Perl is Y2K compliant.

• Perl supports both procedural and object-oriented programming.

• Perl interfaces with external C/C++ libraries through XS or SWIG.

• Perl is extensible. There are over 20,000 third party modules available from the Comprehensive Perl Archive Network (CPAN).

• The Perl interpreter can be embedded into other systems.

• Perl used to be the most popular web programming language due to its text manipulation capabilities and rapid development cycle.

• Perl is widely known as " the duct-tape of the Internet".

• Perl can handle encrypted Web data, including e-commerce transactions.

• Perl can be embedded into web servers to speed up processing by as much as 2000%.

• Perl's mod_perl allows the Apache web server to embed a Perl interpreter.

• Perl's DBI package makes web-database integration easy.

Yes! Perl is a case sensitive programming language.

A Perl identifier is a name used to identify a variable, function, class, module, or other object. A Perl variable name starts with either $, @ or % followed by zero or more letters, underscores, and digits (0 to 9).

Perl has three basic data types − scalars, arrays of scalars, and hashes of scalars, also known as associative arrays.

Scalars are simple variables. They are preceded by a dollar sign ($). A scalar is either a number, a string, or a reference. A reference is actually an address of a variable, which we will see in the upcoming chapters.

Arrays are ordered lists of scalars that you access with a numeric index which starts with 0. They are preceded by an "at" sign (@).

Hashes are unordered sets of key/value pairs that you access using the keys as subscripts. They are preceded by a percent sign (%).

Perl variables do not have to be explicitly declared to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign (=) is used to assign values to variables.

Perl treats same variable differently based on Context, i.e. situation where a variable is being used.

Assignment to a scalar variable evaluates the right-hand side in a scalar context.

Assignment to an array or a hash evaluates the right-hand side in a list context.

Boolean context is simply any place where an expression is being evaluated to see whether it's true or false.

This context not only doesn't care what the return value is, it doesn't even want a return value.

This context only happens inside quotes, or things that work like quotes.

Single quoted string prints the perl variable as a string whereas double quoted string evaluates the variable and used to get the variable's value.

#!/usr/bin/perl

$var = "This is string scalar!";
$quote = 'I m inside single quote - $var';
$double = "This is inside double quote - $var";

$escape = "This example of escape -\tHello, World!";

print "var = $var\n";
print "quote = $quote\n";
print "double = $double\n";
print "escape = $escape\n";

This will produce the following result −

var = This is string scalar!
quote = I m inside single quote - $var
double = This is inside double quote - This is string scalar!
escape = This example of escape - Hello, World!

A literal of the form v1.20.300.4000 is parsed as a string composed of characters with the specified ordinals. This form is known as v-strings.

A v-string provides an alternative and more readable way to construct strings, rather than use the somewhat less readable interpolation form "\x{1}\x{14}\x{12c}\x{fa0}".

It is used to get the current file name.

It is used to get the current line number.

It is used to get the current package name.

To refer to a single element of an array, you will use the dollar sign ($) with the variable name followed by the index of the element in square brackets.

Here is a simple example of using the array variables −

#!/usr/bin/perl

@ages = (25, 30, 40);             
@names = ("John Paul", "Lisa", "Kumar");

print "\$ages[0] = $ages[0]\n";
print "\$ages[1] = $ages[1]\n";
print "\$ages[2] = $ages[2]\n";
print "\$names[0] = $names[0]\n";
print "\$names[1] = $names[1]\n";
print "\$names[2] = $names[2]\n";

When exected, this will produce the following result −

$ages[0] = 25
$ages[1] = 30
$ages[2] = 40
$names[0] = John Paul
$names[1] = Lisa
$names[2] = Kumar

range operator (..) is used to create sequential arrays.

#!/usr/bin/perl

@var_10 = (1..10);
@var_20 = (10..20);
@var_abc = (a..z);

print "@var_10\n";   # Prints number from 1 to 10
print "@var_20\n";   # Prints number from 10 to 20
print "@var_abc\n";  # Prints number from a to z

Here double dot (..) is called range operator. This will produce the following result −

1 2 3 4 5 6 7 8 9 10
10 11 12 13 14 15 16 17 18 19 20
a b c d e f g h i j k l m n o p q r s t u v w x y z

The size of an array can be determined using the scalar context on the array - the returned value will be the number of elements in the array −

@array = (1,2,3);
print "Size: ",scalar @array,"\n";

The value returned will always be the physical size of the array, not the number of valid elements.

push @ARRAY, LIST - Pushes the values of the list onto the end of the array.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# add one element at the end of the array
push(@coins, "Penny");
print "2. \@coins  = @coins\n";

This will produce the following result −

1. @coins = Quarter Dime Nickel
2. @coins = Quarter Dime Nickel Penny

unshift @ARRAY, LIST - Prepends list to the front of the array, and returns the number of elements in the new array.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# add one element at the beginning of the array
unshift(@coins, "Dollar");
print "2. \@coins  = @coins\n";

This will produce the following result −

1. @coins = Quarter Dime Nickel
2. @coins = Dollar Quarter Dime Nickel

pop @ARRAY − Pops off and returns the last value of the array.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# remove one element from the last of the array.
pop(@coins);
print "2. \@coins  = @coins\n";

This will produce the following result −

1. @coins = Quarter Dime Nickel
2. @coins = Quarter Dime

shift @ARRAY − Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# remove one element from the beginning of the array.
shift(@coins);
print "2. \@coins  = @coins\n";

This will produce the following result −

1. @coins = Quarter Dime Nickel
2. @coins = Dime Nickel

You can also extract a "slice" from an array − that is, you can select more than one item from an array in order to produce another array.

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

@weekdays = @days[3,4,5];

print "@weekdays\n";

This will produce the following result -

Thu Fri Sat

splice() function will remove the elements of @ARRAY designated by OFFSET and LENGTH, and replaces them with LIST, if specified. Finally, it returns the elements removed from the array.

splice @ARRAY, OFFSET [ , LENGTH [ , LIST ] ]

Following is the example −

#!/usr/bin/perl

@nums = (1..20);
print "Before - @nums\n";

splice(@nums, 5, 5, 21..25); 
print "After - @nums\n";

This will produce the following result −

Before − 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
After − 1 2 3 4 5 21 22 23 24 25 11 12 13 14 15 16 17 18 19 20

split() splits a string into an array of strings, and returns it. If LIMIT is specified, splits into at most that number of fields. If PATTERN is omitted, splits on whitespace.

split [ PATTERN [ , EXPR [ , LIMIT ] ] ]

Following is the example −

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

print "$string[3]\n";  # This will print Roses
print "$names[4]\n";   # This will print Michael

This will produce the following result −

Roses
Michael

join() function joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns the string.

join EXPR, LIST

Following is the example −

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

$string1 = join( '-', @string );
$string2 = join( ',', @names );

print "$string1\n";
print "$string2\n";
print "$string[3]\n";  # This will print Roses
print "$names[4]\n";   # This will print Michael

This will produce the following result −

Rain-Drops-On-Roses-And-Whiskers-On-Kittens
Larry,David,Roger,Ken,Michael,Tom

The sort() function sorts each element of an array according to the ASCII Numeric standards. This function has the following syntax −

sort [ SUBROUTINE ] LIST

This function sorts the LIST and returns the sorted array value. If SUBROUTINE is specified then specified logic inside the SUBTROUTINE is applied while sorting the elements.

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Before: @foods\n";

# sort this array
@foods = sort(@foods);
print "After: @foods\n";

This will produce the following result −

Before: pizza steak chicken burgers
After: burgers chicken pizza steak

This special variable is a scalar containing the first index of all arrays. Because Perl arrays have zero-based indexing, $[ will almost always be 0. But if you set $[ to 1 then all your arrays will use on-based indexing. It is recommended not to use any other indexing other than zero. However, let's take one example to show the usage of $[ variable −

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Foods: @foods\n";

# Let's reset first index of all the arrays.
$[ = 1;

print "Food at \@foods[1]: $foods[1]\n";
print "Food at \@foods[2]: $foods[2]\n";

This will produce the following result −

Foods: pizza steak chicken burgers
Food at @foods[1]: pizza
Food at @foods[2]: steak

Because an array is just a comma-separated sequence of values, you can combine them together as shown below.

#!/usr/bin/perl

@numbers = (1,3,(4,5,6));

print "numbers = @numbers\n";

This will produce the following result −

numbers = 1 3 4 5 6

Hashes are created in one of the two following ways. In the first method, you assign a value to a named key on a one-by-one basis −

$data{'John Paul'} = 45;
$data{'Lisa'} = 30;
$data{'Kumar'} = 40;

In the second case, you use a list, which is converted by taking individual pairs from the list: the first element of the pair is used as the key, and the second, as the value. For example −

%data = ('John Paul', 45, 'Lisa', 30, 'Kumar', 40);

When accessing individual elements from a hash, you must prefix the variable with a dollar sign ($) and then append the element key within curly brackets after the name of the variable. For example −

#!/usr/bin/perl

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);

print "$data{'John Paul'}\n";
print "$data{'Lisa'}\n";
print "$data{'Kumar'}\n";

This will produce the following result −

45
30
40

You can get a list of all of the keys from a hash by using keys function, which has the following syntax −

keys %HASH

This function returns an array of all the keys of the named hash. Following is the example −

#!/usr/bin/perl 

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);

@names = keys %data;

print "$names[0]\n";
print "$names[1]\n";
print "$names[2]\n";

This will produce the following result −

Lisa
John Paul
Kumar

You can get a list of all of the values from a hash by using values function, which has the following syntax −

values %HASH

This function returns an array of all the values of the named hash. Following is the example −

#!/usr/bin/perl 

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);

@ages = values %data;

print "$ages[0]\n";
print "$ages[1]\n";
print "$ages[2]\n";

This will produce the following result −

30
45
40

Using the exists function, which returns true if the named key exists, irrespective of what its value might be −

#!/usr/bin/perl

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);

if( exists($data{'Lisa'} ) ){
   print "Lisa is $data{'Lisa'} years old\n";
}
else{
   print "I don't know age of Lisa\n";
}

Here we have introduced the IF...ELSE statement, which we will study in a separate chapter. For now you just assume that if( condition ) part will be executed only when the given condition is true otherwise else part will be executed. So when we execute the above program, it produces the following result because here the given condition exists($data{'Lisa'} returns true −

Lisa is 30 years old

You can get the size - that is, the number of elements from a hash by using the scalar context on either keys or values. Simply saying first you have to get an array of either the keys or values and then you can get the size of array as follows −

#!/usr/bin/perl

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);

@keys = keys %data;
$size = @keys;
print "1 - Hash size:  is $size\n";

@values = values %data;
$size = @values;
print "2 - Hash size:  is $size\n";

This will produce the following result −

1 - Hash size: is 3
2 - Hash size: is 3

Adding a new key/value pair can be done with one line of code using simple assignment operator.

#!/usr/bin/perl

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);
@keys = keys %data;
$size = @keys;
print "1 - Hash size:  is $size\n";

# adding an element to the hash;
$data{'Ali'} = 55;
@keys = keys %data;
$size = @keys;
print "2 - Hash size:  is $size\n";

This will produce the following result −

1 - Hash size: is 3
2 - Hash size: is 4

To remove an element from the hash you need to use delete function as shown below in the example−

#!/usr/bin/perl

%data = ('John Paul' => 45, 'Lisa' => 30, 'Kumar' => 40);
@keys = keys %data;
$size = @keys;
print "1 - Hash size:  is $size\n";

# delete the same element from the hash;
delete $data{'John Paul'};
@keys = keys %data;
$size = @keys;
print "2 - Hash size:  is $size\n";

This will produce the following result −

1 - Hash size: is 3
2 - Hash size: is 2

It causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. last statement.

It terminates the loop statement and transfers execution to the statement immediately following the loop. continue statement.

A continue BLOCK, it is always executed just before the conditional is about to be evaluated again.

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed.

The goto LABEL form jumps to the statement labeled with LABEL and resumes execution from there.

The goto EXPR form is just a generalization of goto LABEL. It expects the expression to return a label name and then jumps to that labeled statement.

It substitutes a call to the named subroutine for the currently running subroutine.

Exponent − Performs exponential (power) calculation on operators. Assume variable $a holds 10 and variable $b holds 20 then $a**$b will give 10 to the power 20.

It checks if the value of two operands are equal or not, and returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. Assume variable $a holds 10 and variable $b holds 20 then ($a <=> $b) returns -1.

It returns true if the left argument is stringwise less than the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a lt $b) is true.

It returns true if the left argument is stringwise greater than the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a gt $b) is false.

It returns true if the left argument is stringwise less than or equal to the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a le $b) is true.

It returns true if the left argument is stringwise greater than or equal to the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a ge $b) is false.

It returns true if the left argument is stringwise equal to the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a eq $b) is false.

It returns true if the left argument is stringwise not equal to the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a ne $b) is true.

It returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. Assume variable $a holds "abc" and variable $b holds "xyz" then ($a cmp $b) is -1.

Exponent AND assignment operator, Performs exponential (power) calculation on operators and assign value to the left operand. $c **= $a is equivalent to $c = $c ** $a

It encloses a string with-in single quotes. q{abcd} gives 'abcd'

It encloses a string with-in double quotes. qq{abcd} gives "abcd"

It encloses a string with-in invert quotes. qx{abcd} gives `abcd`

Binary operator dot (.) concatenates two strings. If $a="abc", $b="def" then $a.$b will give "abcdef"

The repetition operator x returns a string consisting of the left operand repeated the number of times specified by the right operand. ('-' x 3) will give ---.

The range operator .. returns a list of values counting (up by ones) from the left value to the right value. (2..5) will give (2, 3, 4, 5).

Auto Increment operator increases integer value by one. $a++ will give 11.

Auto Decrement operator decreases integer value by one. $a−− will give 9

The arrow operator is mostly used in dereferencing a method or variable from an object or a class name. $obj−>$a is an example to access variable $a from object $obj.

localtime() function, which returns values for the current date and time if given no arguments.

#!/usr/local/bin/perl
 
@months = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
@days = qw(Sun Mon Tue Wed Thu Fri Sat Sun);

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime();
print "$mday $months[$mon] $days[$wday]\n";

When the above code is executed, it produces the following result −

16 Feb Sat

The function gmtime() works just like localtime() function but the returned values are localized for the standard Greenwich time zone. When called in list context, $isdst, the last value returned by gmtime, is always 0 . There is no Daylight Saving Time in GMT.

localtime() will return the current local time on the machine that runs the script and gmtime() will return the universal Greenwich Mean Time, or GMT (or UTC).

You can use the time() function to get epoch time, i.e. the numbers of seconds that have elapsed since a given date, in Unix is January 1, 1970.

You can use the POSIX function strftime() to format date and time.

The general form of a subroutine definition in Perl programming language is as follows −

sub subroutine_name{
   body of the subroutine
}

The typical way of calling that Perl subroutine is as follows −

subroutine_name( list of arguments );

they can be acessed inside the function using the special array @_. Thus the first argument to the function is in $_[0], the second is in $_[1], and so on.

using scalar(@_), we can get the total number of arguments passed.

The my operator confines a variable to a particular region of code in which it can be used and accessed. Outside that region, this variable cannot be used or accessed.

By default, all variables in Perl are global variables, which means they can be accessed from anywhere in the program.

Lexical variables are private variables created using my operator.

The local is used when the current value of a variable must be visible to called subroutines.

A local just gives temporary values to global (meaning package) variables. This is known as dynamic scoping.

Lexical scoping is done with my operator. A lexical scope is usually a block of code with a set of braces around it, such as those defining the body of the subroutine or those marking the code blocks of if, while, for, foreach, and eval statements. The my operator confines a variable to a particular region of code in which it can be used and accessed. Outside that region, this variable cannot be used or accessed.

There are another type of lexical variables, which are similar to private variables but they maintain their state and they do not get reinitialized upon multiple calls of the subroutines. These variables are defined using the state operator and available starting from Perl 5.9.4.

The context of a subroutine or statement is defined as the type of return value that is expected. This allows you to use a single function that returns different values based on what the user is expecting to receive. For example, the following localtime() returns a string when it is called in scalar context, but it returns a list when it is called in list context.

my $datestring = localtime( time );

In this example, the value of $timestr is now a string made up of the current date and time, for example, Thu Nov 30 15:21:33 2000. Conversely −

($sec,$min,$hour,$mday,$mon, $year,$wday,$yday,$isdst) = localtime(time);

Now the individual variables contain the corresponding values returned by localtime() subroutine.

A Perl reference is a scalar data type that holds the location of another value which could be scalar, arrays, or hashes. Because of its scalar nature, a reference can be used anywhere, a scalar can be used.

You can create a reference for any variable by prefixing it with a backslash as follows −

$scalarref = \$foo;

You can create a reference for any array by prefixing it with a backslash as follows −

$arrayref  = \@ARGV;

You can create a reference for any hash by prefixing it with a backslash as follows −

$hashref   = \%ENV;

You can create a reference for any subrouting by prefixing it with a backslash as follows −

$cref = \&PrintHash;

Dereferencing returns the value from a reference point to the location.

To dereference a reference simply use $, @ or % as prefix of the reference variable depending on whether the reference is pointing to a scalar, array, or hash.

A circular reference occurs when two references contain a reference to each other. You have to be careful while creating references otherwise a circular reference can lead to memory leaks. Following is an example −

#!/usr/bin/perl
my $foo = 100;
$foo = \$foo;
 
print "Value of foo is : ", $$foo, "\n";

When above program is executed, it produces the following result −

Value of foo is : REF(0x9aae38)

Following is the syntax to open file.txt in read-only mode. Here less than < sign indicates that file has to be opend in read-only mode.

open(DATA, "<file.txt");

Here DATA is the file handle which will be used to read the file.

Following is the syntax to open file.txt in writing mode. Here less than > sign indicates that file has to be opend in the writing mode.

open(DATA, ">file.txt") or die "Couldn't open file file.txt, $!";

Following is the syntax to open file.txt in writing mode without truncating it. Here less than +< sign indicates that file has to be opend in the writing mode without truncating it.

open(DATA, "+<file.txt") or die "Couldn't open file file.txt, $!";

To close a filehandle, and therefore disassociate the filehandle from the corresponding file, you use the close function. This flushes the filehandle's buffers and closes the system's file descriptor.

The getc function returns a single character from the specified FILEHANDLE, or STDIN if none is specified.

The read function reads a block of information from the buffered filehandle: This function is used to read binary data from the file.